1. Field of the Invention
The present invention relates to a lead overlay spin valve sensor with antiferromagnetic layers in passive regions for stabilizing a free layer and, more particularly, to such sensors which are highly sensitive and stable even though they have a small track width.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer structure and a ferromagnetic free layer structure. An antiferromagnetic pinning layer interfaces the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer structure is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is parallel to the ABS, is the position of the magnetic moment of the free layer structure when the sense current is conducted through the sensor in the absence of signal fields.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layer structures are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with the pinned and free layer structures. When the magnetic moments of the pinned and free layer structures are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layer structures. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in the resistance of the sensor as the magnetic moment of the free layer structure rotates from a position parallel with respect to the magnetic moment of the pinned layer structure to an antiparallel position with respect thereto and R is the resistance of the sensor when the magnetic moments are parallel.
In addition to the spin valve sensor the read head includes nonconductive nonmagnetic first and second read gap layers and ferromagnetic first and second shield layers. The spin valve sensor is located between the first and second read gap layers and the first and second read gap layers are located between the first and second shield layers. In the construction of the read head the first shield layer is formed first followed by formation of the first read gap layer, the spin valve sensor, the second read gap layer and the second shield layer. Spin valve sensors are classified as a top or a bottom spin valve sensor depending upon whether the pinning layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic layers that are separated by a coupling layer with magnetic moments of the ferromagnetic layers being antiparallel. Spin valve sensors are still further classified as single or dual wherein a single spin valve sensor employs only one pinned layer and a dual spin valve sensor employs two pinned layers with the free layer structure located therebetween.
A typical spin valve sensor has top and bottom surfaces and first and second side surfaces which intersect the ABS. Prior art read heads employ first and second hard bias layers and first and second lead layers that abut the first and second side surfaces for longitudinally biasing and stabilizing the free layer in the sensor and conducting a sense current transversely through the sensor. The track width of the head is measured between the centers of the side surfaces of the free layer. In an effort to reduce the track width to submicron levels it has been found that the hard bias layers make the free layer magnetically stiff so that its magnetic moment does not freely respond to field signals from a rotating magnetic disk. Accordingly, there is a strong-felt need to provide submicron track width spin valve sensors which are still sensitive to the signals from the rotating magnetic disk along with longitudinal biasing of the free layer transversely so that the free layer is kept in a single magnetic domain state.
The present invention provides a submicron track width bottom spin valve sensor wherein the free layer is highly sensitive to field signals from a rotating magnetic disk even though the free layer is longitudinally biased for stabilization purposes. The spin valve sensor has a transverse length between the first and second side surfaces which is divided into a track width region between first and second passive regions wherein the track width region is defined by the first and second lead layers. The free layer, which is at the top of the sensor, has first and second passive portions in the first and second passive regions wherein the first and second passive portions have first and second top surfaces respectively. As in the prior art, the first and second hard bias layers still abut the first and second side surfaces of the spin valve sensor. The first and second lead layers overlap the first and second top surfaces of the free layer and are electrically connected thereto for conducting the sense current through the sensor. Accordingly, the first and second hard bias layers are located outwardly with respect to the first and second lead layers. A space between the first and second lead layers in a central portion of the spin valve sensor defines the track width of the head and the aforementioned track width region which can be submicron. By locating the first and second hard bias layers remotely from the track width region the hard bias layers do not make the free layer in the track width region insensitive to field signals from the rotating magnetic disk. Unfortunately, however, a portion of the first and second passive portions of the spin valve sensor are not sufficiently biased and are not magnetically stable. Further, these passive portions perform side reading on each side of a track being read by the active portion of the sensor, between the lead layers, which introduces errors into the signal. Still further, the remote location of the hard bias layers may not provide full stabilization of the free layer in the active track width region. This problem has been overcome by providing first and second antiferromagnetic layers which are exchange coupled to the first and second top surfaces respectively of the free layer in the first and second passive portions of the free layer. This exchange coupling orients the magnetic moment of the first and second passive portions of the free layer parallel to the ABS which, by magnetostatic coupling, orients the magnetic moment of the active region of the free layer parallel to the ABS. The first and second antiferromagnetic layers are conductive so that the first and second lead layers can be formed thereon for conducting the sense current through the sensor.
It is important that a blocking temperature of the first and second antiferromagnetic layers be lower than the blocking temperature of the pinning layer. The blocking temperature is the temperature at which the magnetic spins of the antiferromagnetic layer are free to move around when subjected to an extraneous field. The setting of the magnetic spins of the first and second antiferromagnetic layers is subsequent to the setting of the spins of the pinning layer and must not reorient the spins of the pinning layer. Accordingly, if the blocking temperature of the first and second antiferromagnetic layers is less than the blocking temperature of the pinning layer the magnetic spins of the pinning layer can be set by a first step involving heat to the blocking temperature in the presence of a magnetic field perpendicular to the ABS and, after constructing the magnetic head on a wafer, the head may be subjected to heating of the first and second antiferromagnetic layers to their blocking temperature in the presence of a field parallel to the ABS for appropriately setting the magnetic spins of the antiferromagnetic layers.
An object is to provide a submicron spin valve sensor with a highly stabilized free layer which is highly responsive to signals from a rotating magnetic disk.
Another object is to provide the aforementioned sensor wherein the pinning layer is not degraded by a scheme for longitudinally biasing the free layer.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.